1. Field of the Invention
The present invention relates to passive magnet bearings and, more particularly, to using such bearings to support a rotating shaft having a horizontal axis of rotation.
2. Description of Related Art
Motor and generator armatures, flywheel rotors, and other rotatable components have conventionally been supported and constrained against transverse and axial movement by mechanical bearings, such as journal bearings, ball bearings, and roller bearings. Such bearings necessarily involve mechanical contact between the rotating element and the bearing components, leading to well-known problems associated with friction and wear. Even non-contacting bearings, such as air bearings, involve frictional losses that can be appreciable, and are also sensitive to dust particles. In addition, mechanical bearings, and especially air bearings, are poorly adapted for use in a vacuum.
The use of magnetic forces to provide a non-contact, low-friction bearing avoids the drawbacks attendant to mechanical and air bearings, and thus provides an attractive alternative. However, magnetic bearings and suspension elements are subject to the constraints of Earnshaw""s Theorem which, as applied to magnetic apparatus, holds that any magnetic suspension element, such as a magnetic bearing that utilizes static magnetic forces between a stationary and a rotating component, cannot exist in a state of equilibrium against external forces, e.g., gravity. In other words if such a bearing element is designed to be stable against transverse displacements, it will be unstable against axially directed displacements, and vice versa. The assumptions implicit in the derivation of Earnshaw""s Theorem are that the magnetic fields are static in nature, i.e., that they arise from either fixed currents or objects of fixed magnetization, and that diamagnetic bodies are excluded.
As a consequence, magnetic bearings are designed to be stable along at least one axis, for example, their axis of symmetry, and then external stabilizing means are used to ensure their stability along the remaining axes. The stabilizing means referred to could either be mechanical, i.e., ball bearings, or, more commonly, electromagnetic. The latter approach uses position sensors to detect incipient unstable motion of the rotating element and magnetic coils in conjunction with electronic servo amplifiers to provide stabilizing forces to restore the element to its (otherwise unstable) position of force equilibrium. The foregoing is usually designated as an xe2x80x9cactivexe2x80x9d magnetic bearing, in reference to the active involvement of electronic feedback circuitry in maintaining stability
Less common than the servo-controlled magnetic bearings just described are magnetic bearings that use superconductors to provide a repelling force acting against a permanent magnet element in such a way as to levitate that magnet. These bearing types utilize the flux-excluding property of superconductors to attain a stable state by properly shaping the superconductor and the magnet in order to provide restoring forces for displacements in any direction from the position of force equilibrium. Needless to say, magnetic bearings that employ superconductors are subject to the limitations imposed by the need to maintain the superconductor at cryogenic temperatures, as well as limitations on the magnitude of the forces that they can exert.
As may be seen from the foregoing, there presently exists a need in the art for a bearing that is magnetic, yet overcomes the limitations of Earnshaw""s Theorem without the drawbacks and limitations attendant to active or superconducting magnetic bearings. The present invention fulfills this need in the art.
Briefly, the present invention is a passive magnetic bearing composed of two elements, one to levitate a horizontal shaft, and the other to restore the shaft to its equilibrium position if it is displaced transverse to its axial axis of rotation. The levitation element is composed of a pair of arcuate segments composed of ferromagnetic material located within an annular radial-field magnet array. The magnet array is attached to the shaft""s inner circumference and rotates with the shaft. The arcuate segments remain stationary with respect to the shaft. The magnetic field of the radial-field magnet array generates an attractive force between the arcuate ferromagnetic segments and the magnet array. The arcuate segments are positioned so that this attractive force is directed vertically to levitate the shaft, and also in a horizontal transverse direction to center the shaft.
The restorative element is composed of an annular Halbach array and an annular circuit array located concentrically within the Halbach array. The Halbach array is attached to the shaft""s inner circumference, and rotates with the shaft. The circuit array remains stationary relative to the shaft. There is a repulsive force between the Halbach array and the circuit array that is induced when the Halbach array rotates relative to the circuit array. The repulsive force increases exponentially with a decrease in the radial space between the Halbach array and the circuit array, and thus acts to restore the shaft to its equilibrium axis of rotation whenever the shaft is transversely displaced therefrom.
In summary, one element of the bearing levitates and centers the shaft, while the other element restores the shaft to its equilibrium axis of rotation in the event it is displaced transversely therefrom.